Children’s theatre is booming in the UK. But all is not sweetness and light, as the new crop of plays and musicals has traded traditional treacle for an increasingly dark and foreboding view of the world, along with “a suspicion of all those who wield power, which for children of course means grown-ups.” The kids are, of course, eating it up, but their grown-up chaperones may find more disturbing subtext than they are ready to handle.